#include <fstream>
#include <vector>

int main()
{
	std::ifstream input("in.txt");
	std::ofstream output("out.txt");
	std::vector<double> x, y;
	double x1, y1;
	while (input >> x1  >> y1)
	{
		x.push_back(x1);
		y.push_back(y1);
	}
	int n = x.size() - 1;
	double s = 0.0;
	for (int i = 0;  i < n; i++)
	{
		s += x[i] * y[i + 1] - x[i + 1] * y[i]; 
	}
	s += x[n] * y[0] - x[0] * y[n];
	s = fabs(s);
	s *= 0.5;
	output << std::fixed << s;
	return 0;
}